698 



CONCHIFERA. 



going any glandular elaboration, the fluids ab- 

 sorbed to the general current of the circulation. 

 After having thus had all the nutritious ele- 

 ments it contains abstracted, the alimentary 

 mass, having reached the rectum, there com- 

 monly presents itself under the form of minute 

 globules ; it is soon afterwards expelled through 

 the anus. 



Organs of circulation, The organs of cir- 

 culation in the acephalous Mollusca consist 

 of two vascular systems forming together a 

 simple circuit, namely, a ventricle and an 

 arterial system, and a venous system and two 

 auricles. The ventricle in the majority of 

 acephalous mollusca is single, symmetrical, 

 situated in the dorsal median line of the body, 

 and rests upon the rectum, which it embraces 

 in its evolution (g, jig. 347, h, Jig. 348) on 

 every side so closely, that the intestine appears 

 to pass through it. It is to be presumed, 

 however, that the intestine does not pass im- 

 mediately athwart the heart, but that this canal 

 is only embraced so intimately by the central 

 organ of the circulation, that it is impossible to 

 separate without tearing them. The ventricle, 

 which is regular and symmetrical in the greater 

 number of the genera (a, Jig. 349) is irregular 

 and unsymmetrical in the Ostracean family, (a, 

 Jig. 350). It is generally elongated and fusiform ; 



Figs. 349 & 350. 



its parietes are thin, formed of muscular fibres 

 variously interlaced, and often projecting in- 

 ternally. From either extremity issues one of 

 the two main arteries of the body, the one 

 superior giving branches to the whole of the 

 anterior parts of the animal ; the other pos- 



cera, the stomach, liver, intestinal canal, and 

 ovary. Many superficial branches penetrate 

 the mantle, and may be observed ramifying 

 more especially upon the thicker parts which 

 constitute its edges. 



When the back of the animal is very broad, 

 and as a necessary consequence of this struc- 

 ture, the branchiae of one side are at a consi- 

 derable distance from those of the other side, 

 we find, as among the Archidae, that there are 

 then two ventricles (a, a, Jig. 351,) and two auri- 

 cles (6, 6, Jig. 351) to secure the perfect per- 

 formance of the important business of circula- 

 tion . This interesting modification of the organs 

 of circulation is of slight significance as regards 

 the mere results of the function, for it still con- 

 tinues no more than a simple circuit, exactly 

 as if it were effected by a single ventricle. 



The auricles are two in number (6, 6, 

 Jigs. 349 & 351, i, Jig. 348) in the whole 

 of the genera of Conchifera except those of 

 the family of the Ostracea, in which there 

 is no more than a single irregular auricle 

 (b>Jig- 350), just as there is but one ventricle. 

 The most general figure presented by the au- 

 ricles is the triangular. They communicate 

 with the ventricle by one of the angles of the 

 triangles, and they receive the blood of the 

 branchiae by the most extensive of their three 

 sides. These organs are altogether membra- 

 nous ; in their interior, however, we discover, 

 with the aid of the magnifier, a great number 

 of small fibrous fasciculi, by means of which 

 the regular contraction of the ventricles appears 

 to be effected. 



The venous system is of very considerable 

 magnitude. In his magnificent work, Poli* 

 has given a very satisfactory account of its 

 anatomy. It is more particularly remarkable 

 in the Archidae, the Pinna, &c. It is destined 

 to receive the blood of the general circulation ; 

 it is also destined to collect the whole of the 

 fluids absorbed, and to direct these towards 

 the branchial apparatus, in which the blood 

 with these added fluids undergoes a fresh 

 elaboration. It is after having traversed the 

 branchial vessels (c, r,c, c, Jig. 349, 351, j, 

 Jig. 348) that the blood revivified is carried to- 

 wards the auricle by the pulmonary veins, from 

 whence it is sent to the ventricle, and by it forced 

 anew to perform the round of the arterial cir- 

 culation. 



The blood in the Conchiferous mollusks is 

 colourless, or of a bluish white, very different 

 from the hue it presents in the vertebrata ; 

 it is but slightly viscid, and when it coagulates 

 exhibits but a very small quantity of crassa- 

 mentum or solid matter. 



Circulation then is an extremely simple 

 function in the Conchiferous mollusks: an 

 aortic ventricle gives the blood impulse enough 

 to carry it through the two systems of vessels, 

 to expel it from the heart and to bring it back 

 again to the auricle. In other branchiferous 

 animals, the auricle is sometimes adapted to 

 give the blood a new impulse when it is about 

 to pass through the branchiae; here, on the 



terior supplying branches to the principal vis- * Testacea Utriusque Siciliae, fol. 3 torn. 



